NO. 8 MORPHOLOGY OF INSECT SENSE ORGANS SNODGRASS 21 



same varicose structure. It is reasonable' to suppose that they are 

 sensitive to external stimuli, to mechanical stimuli, at least, and pos- 

 sibly to changes of temperature. Sparsely-haired caterpillars react to 

 gentle pressure on the skin between the tactile hairs, and naked, soft- 

 bodied larvae are well known to be highly sensitive to touch anywhere 

 on the body surface, showing that a sense of touch is not dependent 

 on the presence of special tactile organs. Viallanes noted that naked 

 fly larvae possess peripheral cells of Type II, though lacking those of 

 Type I which innervate the body hairs of other species. Since the 

 nerve endings of cells of Type II, however, are apparently all alike, 

 and are not associated with specialized cells of the hypodermis, it 

 may be questioned whether they receive differentiated sensations. Pos- 

 sibly, perceptions received through them, whether of mechanical, 

 thermal, or chemical stimuli, are merely general sensations akin to 

 degrees of pain. 



It is suggested by Orlov (1924) that the skeletal muscles of insects 

 receive a sensory innervation through their tendons from the subhy- 

 podermal nerves. Orlov, however, remarks that there is no litera- 

 ture on the sensory nerves of the skeletal muscles of invertebrates, 

 except for the negative statement of Doflein that no such nerves are 

 present in the Arthropoda. 



The peripheral sensory cells of Type I, the single distal processes 

 of which are non-varicose, non-branched, and go direct to the tac- 

 tile hairs or other cuticular sense organs, are associated with the 

 cells of Type II in hairy larv?e, since they commonly lie beneath the 

 hypodermis, more or less removed from the bases of the sense organs. 

 These cells are usually larger than the others, and have more regular, 

 oval, .or fusiform shapes (fig. 11, SCI). The sense cells of adult in- 

 sects generally lie within the hypodermis (fig. 12) and, in their ori- 

 gin, are clearly modified hypodermal cells. The subhypodermal sense 

 cells of larval insects, and the intrahypodermal sense cells of adults 

 are possibly not of the same origin, but on this subject we have no 

 light at present. 



THE SENSORY INNERVATION OF THE ALIMENTARY CANAL 



There are only two papers, known to the writer, treating of the 

 sensory nerves of the alimentary canal in insects, and the endings 

 of the nerves in the various tissues of the tract. One of these is 

 by Zawarzin (1916), describing the stomatogastric system of Peri- 

 plancta americana; the other is by Orlov (1924) on the innervation 

 of the alimentary tract of Lamellicorn beetle larvae. 



